The present invention relates to a process system for the production of olefins and particularly to processing the charge gas feed to more effectively recover the product and process the by-products.
Ethylene, propylene and other valuable petrochemicals are produced by the thermal cracking of a variety of hydrocarbon feedstocks ranging from ethane to heavy vacuum gas oils. In the thermal cracking of these feedstocks, a wide variety of products are produced ranging from hydrogen to pyrolysis fuel oil. The effluent from the cracking step, commonly called charge gas or cracked gas, is made up of this full range of materials which must then be separated (fractionated) into various product and by-product streams followed by reaction (hydrogenation) of at least some of the unsaturated by-products.
The typical charge gas stream, in addition to the desired products of ethylene and propylene, contains C.sub.2 acetylenes, C.sub.3 acetylenes and dienes and C.sub.4 and heavier acetylenes, dienes and olefins as well as a significant quantity of hydrogen. In the majority of prior processes, the C.sub.2 acetylenes and C.sub.3 acetylenes and dienes and the C.sub.5 and heavier dienes, acetylenes and olefins are catalytically hydrogenated in fixed bed reactors using a series of commercially available catalysts. In a growing number of applications, the C.sub.4 acetylenes, dienes, and olefins are also catalytically hydrogenated in fixed bed reactors. These separate hydrogenation steps take place in one of two process sequences. In a typical prior art process, the charge gas is compressed to between 2.76 and 4.14 MPa (400 and 600 psia). It is then progressively chilled condensing the C.sub.2 and heavier components. Hydrogen is cryogenically recovered and methane is fractionated out of the stream. The remaining C.sub.2 and heavier stream enters a series of fractionation towers. The first tower, the deethanizer, produces an overhead stream containing the C.sub.2 acetylenes, olefins, and paraffins. This stream is sent to a fixed bed, vapor phase reactor where the C.sub.2 acetylene is selectively hydrogenated using the hydrogen cryogenically separated earlier from the charge gas stream.
The second tower in this sequence, the depropanizer, produces an overhead stream containing the C.sub.3 acetylenes, dienes, olefins and paraffins. This stream is sent to a fixed bed, vapor or liquid phase reactor where the C.sub.3 acetylenes and dienes are selectively hydrogenated using the hydrogen cryogenically separated earlier from the charge gas stream.
The third tower, the debutanizer, produces an overhead stream containing the C.sub.4 acetylenes, dienes, olefins, and paraffins. This stream is then sent either to battery limits as a final product or to a fixed bed, liquid phase reactor where the dienes, acetylenes, and in some instances the olefins are hydrogenated using the hydrogen cryogenically recovered previously from the charge gas.
The bottoms of the third tower contains the C.sub.5 and heavier dienes, acetylenes, olefins and paraffins. This stream is sent to a series of two fixed bed, liquid phase reactors. In the first, the acetylenes and dienes are catalytically hydrogenated. The olefins are catalytically hydrogenated in the second reactor. Both reactors utilize the hydrogen cryogenically recovered previously from the charge gas. In some applications, the third tower produces an overhead stream containing both the C.sub.4 and C.sub.5 acetylenes, dienes, olefins, and paraffins. These are hydrogenated as discussed previously for the C.sub.4 's alone, in a single fixed bed, liquid phase reactor. The C.sub.6 and heavier dienes, acetylenes, olefins and paraffins exit in the bottoms of the third tower and are hydrogenated as discussed previously in two fixed bed, liquid phase reactors.
In a variation of the typical process just described, the cracked gas is compressed to between 2.07 and 3.45 MPa (300 and 500 psia) and sent to a fractionation tower. The overhead of the tower is the C.sub.3 and lighter portion of the charge gas. It is sent to a series of fixed bed, vapor phase reactors where the C.sub.2 acetylene and a portion of the C.sub.3 acetylenes and dienes are hydrogenated using a small portion of the hydrogen contained in the C.sub.3 and lighter stream. The unhydrogenated portion of the C.sub.3 acetylenes and dienes as well as the C.sub.4 and heavier acetylenes, dienes, and olefins are hydrogenated in a fashion similar to that described above. In many new olefin plants, butadienes are hydrogenated to olefins or butadienes and butenes are totally hydrogenated to butanes. In some cases, the saturated C.sub.4 's, and in some instances the saturated C.sub.5 's also, are recycled to the cracking heaters.
While widely practiced, the typical processes described above have a number of disadvantages. Where the unsaturated C.sub.3 's (methyl acetylene and propadiene), C.sub.4 's and gasoline (including the C.sub.5 's) are being hydrogenated, at least three separate fixed bed reactors are required. If gasoline is being hydrogenated in two stages, the number of fixed bed reactors is four. This number of fixed bed reactors contributes significantly to the capital cost of the system and to the operational complexity. Even when a system is used which processes the C.sub.4 and C.sub.5 unsaturates together, rather than the C.sub.4 's separately and the C.sub.5 's together with the gasoline, one less fractionating tower is required but the number of hydrogenation reactors remains the same.